Multilayer optics for 6.7 nm wavelength

Optics for extreme ultraviolet radiation of 6.5-6.9 nm wavelength are of interest for many applications, such as photolithography, x-ray free electron lasers, x-ray spectroscopy or space research. Photolithography using a wavelength of ~6 nm is considered to be a potential extension of the currently introduced 13.5 nm, or EUV lithography. EUVL poses extremely high requirements on the quality of the multilayer optics. The performance of a single multilayer mirror (MLM) will affect the throughput of a multi-mirror optical system, proportionally to the power of the number of mirrors. The currently highest reflectivity of a single MLM of about 50 % is significantly lower than a desired reflectance of above 70 %. To reach this performance we first optimize the multilayer design by selecting the materials that shows the best optical contrast. The deposition process should be optimized to enable the lowest possible interface roughness to maximize layer contrast. The best material combination for 6.7 nm wavelength is based on lanthanum as a reflector and boron as a spacer material. In this presentation we will discuss the optimization of the optical design of La/B MLM, optimization of the deposition process and the analysis of the MLMs.